1. Field of the Invention
The invention relates to a system for controlling and sensing the operation of an AC induction drive motor and more specifically to a system for controlling and sensing the pulse width modulated (PWM) signal that powers an AC induction drive motor in a treadmill.
2. Description of Related Art
Many treadmills have walk belts powered by AC drive motors. These AC drive motors rotate pulleys and walk belts at speeds that depend on the frequency of the AC power provided to the motor.
When a person walks or runs on a treadmill the impact of the person's foot on the walk belt pushes the walk belt into frictional contact with the treadmill deck of the treadmill. This frictional contact between the walk belt and the treadmill deck resists the smooth movement of the walk belt and creates an increased resistance to the rotation of the pulleys and motor. A number of solutions have been proposed to reduce the amount of friction between the walk belt and treadmill deck. One commonly used approach involves coating the treadmill deck and/or walk belt with a wax or lubricous coating. A difficulty with this approach is that the wax or lubricous coating has a tendency to wear off after extended use. The amount of frictional resistance to the movement of the walk belt on the treadmill deck also varies greatly between users and is a function of the person's stride, weight and various other factors.
We have found that when a person is walking or running on the treadmill at a relatively continuous pace, the amount of frictional resistance to the rotation of the walk belt is nearly sinusoidal in nature. The frictional resistance is at its maximum when the person is planting his foot and is at its minimum or near zero when the person is pushing off the walk belt surface with his foot. This periodic increase in frictional resistance to the movement of the walk belt translates into a variable load applied to the AC drive motor of the treadmill. Because the frictional resistance of a person walking or running on the walk belt at a relatively constant speed is nearly sinusoidal in nature, the load placed on the AC drive motor by the person walking or running on the walk belt is also nearly sinusoidal in nature. The adjustment to the application of a variable load to the AC drive motor is important in maintaining the speed of treadmills relatively constant and is particularly noticeable when a heavier person plants his foot on the walk belt and the speed of the walk belt is at a minimum or lower speed.
The increased load on the AC drive motor caused by the user walking on the walk belt causes a drop in the high voltage DC supply across the filter capacitor. This reduction in the high voltage DC supply reduces the available voltage to power the motor which causes the performance and torque produced by the electronic drive to deteriorate.
It is highly desirable for the AC drive motor to maintain a constant speed at a given AC power frequency despite the varying loads placed on the drive motor during use of the treadmill. The constant speed of the AC drive motor requires the maintenance of a constant magnetic flux density in the windings of the AC drive motor.
Some currently available AC drive motors receive power through a pulse width modulated (PWM) DC signal. By controlling the duty cycles of the pulses in the PWM signals, signals are produced that, when applied to the AC motor, act like AC signals of any desired frequency. This allows the AC drive motor to operate at a desired speed by setting the duty cycle of the PWM signals to produce AC signals at a frequency determined to produce the desired speed of the AC motor.
A prior art system for producing a PWM signal to drive an AC drive motor is disclosed in U.S. Pat. No. 5,140,248 granted to Rowan et al. In this system, the motor control or drive includes a power section that receives power at a line frequency of 60 Hz from a three phase AC power source. The three phases of the power source are connected to an AC-DC power converter in the power section of the drive. The AC-DC power converter passes the AC signal through a full-bridge rectifier system that converts the AC signal into a near DC signal on a DC bus that connects to power inputs on a pulse width modulation voltage inverter which completes the power section of the drive. The PWM inverter includes a group of switching elements which are turned on and off to convert the DC voltage to pulses of constant magnitude.
The pulse train pattern from the PWM inverter is characterized by a first set of positive going pulses of constant magnitude but of varying pulse width and by a second set of negative going pulses of constant magnitude but of varying pulse width. The RMS value of the pulse train pattern approximates one cycle of a sinusoidal signal which is characteristic of an AC wave form. The pattern is repeated to generate additional cycles of the AC wave form.
To control the frequency and, magnitude of the resultant AC power signals to the motor, AC inverter phase control signals are applied to the PWM inverter. The PWM voltage inverter receives three balanced AC inverter phase control signals which vary in phase by 120 degrees, and the magnitude and the frequency of these signals determines the pulse widths and the number of pulses in the pulse trains which are applied to the terminals of the motor. The inverter phase control signals are produced as a result of a 2 phase to 3 phase conversion which is accomplished with a 2 to 3 phase converter.
U.S. Pat. No. 5,140,248 granted to Rowan et al., also discloses a motor controller system which attempts to maintain current constant for a fixed load by limiting the current provided to the motor. As shown in FIG. 1, a microprocessor-based PWM generator produces PWM signals that control switches passing or not passing DC power to the AC motor in response to the PWM signals from the PWM generator. The current at each of the switches is sensed, passed through an A-to-D converter and presented to the microprocessor. As the current at the switches drops below an acceptable level, the microprocessor causes the PWM signals to that switch to have a longer duty cycle so that the input signal is adjusted to regulate the current to the motor.
As described above, various PWM controller systems for AC motors are generally known. Some of these systems sense the PWM signal as it is presented to the drive motor. These systems pass information about the sensed condition of the PWM signal to the PWM generator to modify the PWM signal. However, these systems do not sense the condition of the bus voltage and do not vary the PWM signal directly to account for the variation in the bus voltage.
When the AC drive motor has a load applied to it, the DC bus voltage drops. Unless the PWM signal is modified, there is insufficient energy provided to the drive motor to maintain a constant speed. Consequently, it is desirable to create a system that senses the load placed on the AC drive motor and adjusts the PWM signal accordingly to produce a near constant drive motor speed and a corresponding constant treadmill belt speed for treadmills despite varying loads.
One approach to solving this problem is to use line voltage to determine the voltage supplied to the AC drive motor through the PWM signal. The difficulty with this approach is that the line voltage varies considerably from place to place and even from time to time at the same place. In the U.S. the line voltage typically varies from about 100 volts to about 130 volts, a 30% variation or between about 99 V to 132 V for a 33% variation. In the PWM system described above, this variation in line voltage produces a variation in the bus voltage supplied to the PWM drives of about 280 volts for a 99-volt line to about 368 volts, less bridge drops and bus sag, for a 130-volt line or 373 volts for a 132 volt line. The bus voltage is used to determine the amplitude of the PWM signal. To maintain a constant magnetic flux density in the windings of the AC drive motor despite the load placed on the AC drive motor by a person walking or running on the treadmill belt, the PWM signal corresponding to the 99-volt line must be different from the PWM signal corresponding to a line voltage of about 115 volts.
Although it may be possible to pass a rectified and filtered signal through a voltage regulator to provide a non-varying voltage to the PWM drivers and subsequently to the AC drive motor, the cost and the complexity of such a system renders this type of approach impractical for large scale manufacture or for use in relatively inexpensive applications. For example, in treadmills, a current of 10 amps is frequently drawn by the AC drive motor. Even at the low voltage of 99-volt AC described above, a maximum of about 2000 watts of electrical power is used by the AC drive motor. Voltage regulators capable of handling 2000 watts are expensive and typically require a large number of components, any one of which may fail or disable the voltage regulator during use.
It is, therefore, an object of the invention to provide a method and device for maintaining the flux density of a drive motor constant through drive voltage PWM generators that are inexpensive to manufacture and reliable.
It is a further object of the invention to provide a device for monitoring or regulating the flux density of a drive motor with a PWM driver that is reliable.